Re: [RFC PATCH v11 10/29] mm: Add AS_UNMOVABLE to mark mapping as completely unmovable

[Kirill A . Shutemov]

On Tue, Jul 25, 2023 at 01:51:55PM +0100, Matthew Wilcox wrote:
> On Tue, Jul 25, 2023 at 01:24:03PM +0300, Kirill A . Shutemov wrote:
> > On Tue, Jul 18, 2023 at 04:44:53PM -0700, Sean Christopherson wrote:
> > > diff --git a/mm/compaction.c b/mm/compaction.c
> > > index dbc9f86b1934..a3d2b132df52 100644
> > > --- a/mm/compaction.c
> > > +++ b/mm/compaction.c
> > > @@ -1047,6 +1047,10 @@ isolate_migratepages_block(struct compact_control *cc, unsigned long low_pfn,
> > >  		if (!mapping && (folio_ref_count(folio) - 1) > folio_mapcount(folio))
> > >  			goto isolate_fail_put;
> > >  
> > > +		/* The mapping truly isn't movable. */
> > > +		if (mapping && mapping_unmovable(mapping))
> > > +			goto isolate_fail_put;
> > > +
> > 
> > I doubt that it is safe to dereference mapping here. I believe the folio
> > can be truncated from under us and the mapping freed with the inode.
> > 
> > The folio has to be locked to dereference mapping safely (given that the
> > mapping is still tied to the folio).
> 
> There's even a comment to that effect later on in the function:
> 
>                         /*
>                          * Only pages without mappings or that have a
>                          * ->migrate_folio callback are possible to migrate
>                          * without blocking. However, we can be racing with
>                          * truncation so it's necessary to lock the page
>                          * to stabilise the mapping as truncation holds
>                          * the page lock until after the page is removed
>                          * from the page cache.
>                          */
> 
> (that could be reworded to make it clear how dangerous dereferencing
> ->mapping is without the lock ... and it does need to be changed to say
> "folio lock" instead of "page lock", so ...)
> 
> How does this look?
> 
>                         /*
>                          * Only folios without mappings or that have
>                          * a ->migrate_folio callback are possible to
>                          * migrate without blocking. However, we can
>                          * be racing with truncation, which can free
>                          * the mapping.  Truncation holds the folio lock
>                          * until after the folio is removed from the page
>                          * cache so holding it ourselves is sufficient.
>                          */
> 

Looks good to me.

-- 
  Kiryl Shutsemau / Kirill A. Shutemov